Fructose makes rats dumber. What sugars should we avoid?

Soft drinks are a source of fructose, linked to impaired learning in a rat… (Scott Olson )

Rats fed fructose-laced drinking water for six weeks performed more slowly in a maze-navigating task, UCLA researchers have found. (Read this L.A. Times opinion article.) They think the effect is due to changes in the way the brain responds to insulin as a result of exposure to fructose.

“Our study shows that a high fructose diet harms the brain as well as the body,” study senior author and UCLA professor Fernando Gomez-Pinilla said in a release about the finding, which was published in the Journal of Physiology(postdoc Rahul Agrawal was first author). “We’re ... concerned about high-fructose corn syrup, which is added to manufactured food products as a sweetener and preservative.”

This is not the first time that fructose has been implicated in causing adverse health effects. Rodent studies have found that the sugar contributes to obesity, diabetes and blood-fat disturbances. Human studies suggest that when consumed in large amounts it raises blood triglycerides and cause the liver to become more sluggish in responding to insulin.

And now the brain.

Still the study shouldn't be interpreted as a specific indictment of high-fructose corn syrup, Gomez-Pinilla stressed in a telephone interview. We OD plenty on added regular sugar, the white stuff. Americans should cut back on all of it.

Sucrose, derived from cane or beets, consists of fructose and glucose linked together. When digested in the gut, it yields them in equal proportion.

High-fructose corn syrup contains about 55% fructose and 45% glucose.

So, 50-50 and 55-45 that isn’t much of a difference.

Yet here’s a funny thing: As the drum beat against high-fructose corn syrup has intensified, more and more companies are making a song and dance about the fact that their product contains added sucrose, a movement that thrills the Sugar Assn.

Who would have thought the day would come that added sugar would be trumpeted as a health food?

Even juice deserves a bad rap, some nutrition scientist say. Juices contain plenty of fructose (fructose is “fruit sugar,” in fact). In this article by Karen Kaplan, one scientist dubbed it “soda in disguise.”

Still, because high-fructose corn syrup is fairly inexpensive, it has been added to many foods, Gomez-Pinilla notes, upping the potential sugar dose we get compared with days of yore.

In addition, he said, high-fructose corn syrup comes in various forms, some of them higher than others. Indeed, a study from USC found that some sodas don’t contain high-fructose corn syrup -- they contain something called “really high-fructose corn syrup” that is 65% fructose in content.

Gomez-Pinilla pointed us to a Princeton study that found rats gained more weight on high-fructose corn syrup than they did on table sugar. They also built up more abdominal fat and had higher blood triglyceride levels.

How might that work? It's possible that high-fructose corn syrup delivers a faster rush of fructose to the body than sucrose, Gomez-Pinilla said, because the fructose is already in the syrup in free form -- it doesn't have to be broken down. That might mean it has different metabolic effects. But nobody really knows yet, he said.

Since we don’t know what’s what, maybe the smart thing is just to avoid eating foods with too many added sugars, period. A growing number of nutrition researchers think the sweet stuff is one of the biggest nutritional detriments to our health.

Want to know which foods are high in added sugars? Check out this tally by freelance writer Chris Woolston, published in the L.A. Times’ Saturday section.

In the study by Gomez-Pinilla and Agrawal, adding a mix of omega-3 fats in the form of flax seed and DHA seemed to protect against the brain-dulling effects of fructose. His and other studies are moving in that direction, he said -- toward looking at diets in combinatorial ways.